PORTS AND BUSSES

This is not a tale about the N.Y. Port Authority and Greyhound. It is an article about the doorways into your computer and the highways and by ways of the mother board.

Starting with the external ports, we have:

Serial Ports

Commonly used to attach peripheral devices such as mice, digital cameras, and external modems. Due to the use of the last device type, they are also referred to as COM (communication) ports. It is denoted as a Serial Port since the data is transferred bit-by-bit. The connection on the back of your computer case is a 9-pin (male) DB plug. The cable connection is referred to as an RS (Recommended Standard) 232. This standard called for a maximum transmission rate of 20 Kbps and a 50 foot limit on cable length. A newer standard, RS-422 allows for higher rates and longer cables with more interference protection than RS-232. It is backward compatible with the older standard.

Parallel Ports

This 25-pin connection is also called the printer port for its most common use. The 25 lines consist of 17 for signals and 8 for grounds. The 17 signal lines are further grouped into Control, Status, and Data. The Control group is used for initializing tasks such as hand-shaking, establishing communication between the computer and the printer. The Data group is for sending commands to the printer. Examples are orientation in portrait (taller than wider) or landscape (the inverse), number of copies, fonts to use, margins, etc. The Status group was introduced with the bidirectional port, which allowed the printer to communicate back to the PC. Messages could then modify printing by sending signals such as Out of Paper, Busy Printing, and low on ink. Epson Co. originally designed the port to transmit at a maximum of 150 Kbps. Known as the Centronics standard. EPP ( Enhanced Parallel Port ) and ECP ( Extended Capability Port ) standards increased transmission to between 2 Mbps and 4 Mbps, with cable length as a factor. These IEEE ( Institute of Electrical and Electronic Engineers) 1284 standards support the bidirectional communication.

SCSI Ports

Small Computer System Interface Ports can support up to 7 devices (plus the host – the computer itself). To identify a device, setup assigns an ID to each peripheral. If simultaneous requests for service are sent to the computer, the higher ID number gets the priority. A port typically supports up to two devices, over two device capability is referred to as a BUS. SCSI Port and SCSI Bus are used interchangeably. A popular use is in a business setting for connecting multiple hard drives. SCSI hard drives command a premium price and run at 10,000 rpm or higher.

SCSI-3, aka Wide Ultra2 SCSI, features a 16-bit bus, fiber optic cable capability and a high speed connection at 80 Mbps. Prior to the popularity of USB , most SCSI cards in home PC's were the interface between scanners and the PC.

PS/2

In 1987, IBM introduced the small form factor for mouse and keyboard connections, freeing up the serial port(s) for other devices. The connector is a round, 6-pin mini DIN (Deutsche Indsustrinorm) plug. Commands are sent to the mouse or keyboard device controller on the motherboard which, in turn, sends an interrupt to the CPU. Note that most instructions are sent on “key-up,” release of a key or mouse button. Exceptions are things like character repeats from the keyboard.

USB

The Universal Serial Bus is major milestone in PC technology. Starting around 1996, the now ubiquitous connection with Plug'n'Play and hot swapping allows for adding and removing devices without opening the PC case or turning off and rebooting the computer. USB 1.1 offered connection speeds of 12 Mbps for high speed devices and 1.5 Mbps for mice and joysticks. It can accommodate up to 127 peripherals, daisy chained to a single port. However, since the connection also typically carries the power for the device, such a count is not practical without an external power source – such as a hub .

USB 2.0 has become the new standard. It runs at a maximum of 480 Mbps and is backward compatible with USB 1.1 devices. Later printers, scanners, and digital cameras are frequently USB connected. Microsoft's SP-1 (Service Pack One) for Windows XP supports USB 2.0.

FireWire

A variation of USB, this IEEE 1394 standard is most popular with digital camcorders, digital VCRs, and digital audio devices. Apple Computer owns the trademark on the term, FireWire. It is also referred to as i-Link and Lynx. It supports up to 63 devices at a maximum speed of 400 Kbps (800 Kbps for the second generation). In practice, FireWire bus speed outperforms USB 2.0 by a small margin.

Where do they go?

This is a stylized representation of a generic midsized tower.

Your computer may have additional or different components such as extended audio capabilities and alternate monitor connections.

BUSSES

Internal interfaces connect PC Cards and devices (such as the Floppy, the Hard Drive, CD, and DVD drives) to the motherboard via slots and cables. There is quite a history of development, which we will skip except for some high points.

IDE

Integrated Drive Electronics interface was an early standard for connection between the motherboard and hard drives. It could accommodate up to four devices via a flat ribbon cable, two devices per cable. The Integrated term meant the communication was direct from the hard drive to the motherboard, without an intervening PC Card.

Communication between the PC and a device is by way of channels , each channel requiring an IRQ (Interrupt ReQuest) and two I/O (Input/Output) addresses. Some examples of reserved IRQs are:

IRQ 0 System timer

IRQ 1 Keyboard

IRQ 12 Mouse

IRQ 14 Primary IDE Channel

IRQ 15 Secondary IDE Channel

To see a complete list of the IRQs used on your computer hit Start | Run and type in msinfo32.exe and <Enter>. This will open the System Information dialog box. Now select Hardware Resources and IRQs . This procedure is for Windows XP. For other OS's (Operating Systems), use Help to find info on IRQs.

EIDE

The Enhanced Integrated Device Electronics bus is an extension of the IDE introduced in 1994. It is also known as Fast ATA (Advanced Technology Attachment), Ultra DMA (Direct Memory Access), and Fast IDE. This standard supports two devices on a channel (IDE can have only one per channel) by designating one as the Master and a second as a Slave. The new bus supported HDs larger than 8 GB. In 1997, EIDE-ATA33 raised the transfer rate from 16.7 to 33 MBps still using a flat cable with 40 conductors. In 1997, ATA-66 upped the transfer rate to 66 MBps but required an 80 conductor flat cable with 40 additional grounds. In general, higher speed devices are more susceptible to noise.

Serial ATA

This is a new bus on the scene, not yet widely available. It is destined to replace all the drive parallel buses. Beyond the speed increase, the form factor of a round cable replacing the flat ribbons will allow for more compact cases with improved cooling and neater layouts. Introduced with a speed of 1.5 Gbps (equivalent to 150 MBps), on April 20, 2004 a transfer rate of 3.0 Gbps (about 300 MBps) was achieved. Expectations for this new technology is a life of 10 years with an with an attainable speed of 6.0 Gbps. On the same date, new cable and connector specifications were approved for external storage devices. Also approved was development for multi-lane cabling for multiple internal devices.

ISA

This Industry Standard Architecture, invented in the early 80's, is pretty much “old hat” these days. Colored black, the slots accept 8-bit and 16-bit cards for speeds of 7.9 and 15.9 MBps, respectively. They are used primarily for legacy devices, such as a SCSI card for an older scanner.

PCI

The white Peripheral Component Interconnect slots have supplanted the ISA in most cases. They are capable of supporting many cards including network, SCSI, and some video cards. The interface can handle both 32-bit and 64-bit cards, has the high bandwidth for such cards, and supports Plug and Play with its automatic configuration for new devices. Requests for service are via interrupts assigned to an IRQ with the ability to share an IRQ with two devices.

AGP

This stands for Accelerated Graphics Port and is a slot on the motherboard for high-level video cards. It is colored brown to distinguish it from the ISA and PCI slots. The bus communicates directly with main memory, reducing the load on the PCI bus. High end video cards contain special video memory to directly output graphic elements, further relieving system resources. In 1999, AGP 4X was introduced with a transfer rate of 1024 MBps.